Thermally processable imaging elements, including films and papers, for producing images by thermal processing are well known. These elements include photothermographic elements in which an image is formed by imagewise exposure of the element to light followed by development by uniformly heating the element. These elements also include thermographic elements in which an image is formed by imagewise heating the element. Such elements are described in, for example, Research Disclosure, June 1978, Item No. 17029 and U.S. Pat. Nos. 3,080,254, 3,457,075 and 3,933,508.
The aforesaid thermally processable imaging elements are often provided with an overcoat layer and/or a backing layer, with the overcoat layer being the outermost layer on the side of the support on which the imaging layer is coated and the backing layer being the outermost layer on the opposite side of the support. Other layers which are advantageously incorporated in thermally processable imaging elements include subbing layers and barrier layers.
To be fully acceptable, a protective overcoat layer for such imaging elements should: (a) provide resistance to deformation of the layers of the element during thermal processing, (b) prevent or reduce loss of volatile components in the element during thermal processing, (c) reduce or prevent transfer of essential imaging components from one or more of the layers of the element into the overcoat layer during manufacture of the element or during storage of the element prior to imaging and thermal processing, (d) enable satisfactory adhesion of the overcoat to a contiguous layer of the element, and (e) be free from cracking and undesired marking, such as abrasion marking, during manufacture, storage, and processing of the element.
A backing layer also serves several important functions which improve the overall performance of thermally processable imaging elements. For example, a backing layer serves to improve conveyance, reduce static electricity and eliminate formation of Newton Rings.
A particularly preferred overcoat for thermally processable imaging elements is an overcoat comprising poly(silicic acid) as described in U.S. Pat. No. 4,741,992, issued May 3, 1988. Advantageously, water-soluble hydroxyl-containing monomers or polymers are incorporated in the overcoat layer together with the poly(silicic acid). The combination of poly(silicic acid) and a water-soluble hydroxyl-containing monomer or polymer that is compatible with the poly(silicic acid) is also useful in a backing layer on the side of the support opposite to the imaging layer as described in U.S. Pat. No. 4,828,971, issued May 9, 1989.
One of the most difficult problems involved in the manufacture of thermally processable imaging elements is that the protective overcoat layer typically does not exhibit adequate adhesion to the imaging layer. The problem of achieving adequate adhesion is particularly aggravated by the fact that the imaging layer is typically hydrophobic while the overcoat layer is typically hydrophilic. One solution to this problem is that described in U.S. Pat. No. 4,886,739, issued Dec. 12, 1989, in which a polyalkoxysilane is added to the thermographic or photothermographic imaging composition and is hydrolyzed in situ to form an R.sub.x Si(OH).sub.4-x moiety which has the ability to crosslink with binders present in the imaging layer and the overcoat layer. Another solution to the problem is that described in U.S. Pat. No. 4,942,115, issued Jul. 17, 1990, in which an adhesion-promoting layer, in particular a layer composed of an adhesion-promoting terpolymer, is interposed between the imaging layer and the overcoat layer.
U.S. Pat. No. 4,828,971 explains the requirements for backing layers in thermally processable imaging elements. It points out that an optimum backing layer must:
To meet all of these requirements with a single layer has proven to be extraordinarily difficult. While the backing layer of the '971 patent has excellent performance characteristics, its electrical conductivity is highly dependent on humidity. Under the very low humidity conditions involved in the high temperature processing chambers employed with thermally processable imaging elements, its conductivity is much too low to provide good protection against the buildup of static charge. Static charge can lead to static marking in the processed film, poor transport through processing equipment, and sticking together of processed sheets of film ("static cling").
In copending commonly assigned U.S. patent application Ser. No. 071,806, which is referred to hereinabove, separate backing and electroconductive layers are provided to meet the very stringent requirements of the thermal processing art. In particular, application Ser. No. 071,806 describes the use of an inner electroconductive layer in which the conductive properties are independent of humidity and the use of a separate backing layer which provides the other necessary features which enable effective use of the element. Use of separate backing and electroconductive layers enables each of them to be specifically tailored to meet the specific requirements for which the layer is intended. However, the need to provide separate electroconductive and backing layers increases both manufacturing complexity and cost. In addition, while the use of separate electroconductive and backing layers provides good protection against static-related problems, it is more effective to utilize an electroconductive material in a surface layer rather than in an interior layer that is overcoated with an insulating material (that is, the conductive layer is a "buried" layer). In the former case it is possible for surface charges to be dissipated by conduction of charge across the conductive surface to a ground plane. A buried conductive layer may only transform free charge to polar charge (the term "polar charge" refers to the presence of equal and opposite charge on the two surfaces of a dielectric sheet) by imaging (with an equal and opposite charge) the triboelectric charge created on the contacted surface. This charge imaging process can reduce the distance at which an electric field can be measured (for example, with a field meter) to a distance on the order of the thickness of the imaging element. Thus for example, a dirt particle at a long distance from the imaging element is unlikely to be attracted to the film as a result of electrostatic attraction. However, foreign objects or other sheets of film that are in direct contact with the imaging element containing the image charge are very likely to be subject to the electric field. Thus charge dissipation via a conductive outermost layer is the most effective way to provide protection against static-related problems. It is toward the objective of providing a surface conductive layer that retains all other desirable features required for thermographic or photothermographic imaging elements that the present invention is directed.